((FIGURE 23.) I observed in some parts that the tops of the laminae of the clay-slate (b in Figure 23) under the superficial detritus and soil (a)were bent, sometimes without being broken, as represented in Figure 23, which is copied from one given by Sir H.De la Beche (page 42 "Geological Manual") of an exactly similar phenomenon in Devonshire.Mr.R.A.C.Austen, also, in his excellent paper on S.E.Devon ("Geological Transactions"volume 6 page 437), has described this phenomenon; he attributes it to the action of frosts, but at the same time doubts whether the frosts of the present day penetrate to a sufficient depth.As it is known that earthquakes particularly affect the surface of the ground, it occurred to me that this appearance might perhaps be due, at least at Concepcion, to their frequent occurrence; the superficial layers of detritus being either jerked in one direction, or, where the surface was inclined, pushed a little downwards during each strong vibration.In North Wales I have seen a somewhat analogous but less regular appearance, though on a greater scale ("London Philosophical Magazine" volume 21 page 184), and produced by a quite different cause, namely, by the stranding of great icebergs; this latter appearance has also been observed in N.America.)In certain spots large quartz veins were numerous, and near them, the cleavage, as was the case with the foliation of the schists in the Chonos Archipelago, became extremely tortuous.
At the northern end of Quiriquina Island, in the Bay of Concepcion, at least eight rudely parallel dikes, which have been guided to a certain extent by the cleavage of the slate, occur within the space of a quarter of a mile.They vary much in composition, resembling in many respects the dikes at Low's Harbour: the greater number consist of feldspathic porphyries, sometimes containing grains of quartz: one, however, was black and brilliant, like an augitic rock, but really formed of feldspar; others of a feldspathic nature were perfectly white, with either an earthy or crystalline fracture, and including grains and regular octagons of quartz;these white varieties passed into ordinary greenstones.Although, both here and at Low's Harbour, the nature of the rock varied considerably in the same dike, yet I cannot but think that at these two places and in other parts of the Chonos group, where the dikes, though close to each other and running parallel, are of different composition, that they must have been formed at different periods.In the case of Quiriquina this is a rather interesting conclusion, for these eight parallel dikes cut through the metamorphic schists in a N.W.and S.E.line, and since their injection the overlying cretaceous or tertiary strata have been tilted (whilst still under the sea) from a N.W.by N.and S.E.by S.line; and again, during the great earthquake of February 1835, the ground in this neighbourhood was fissured in N.W.and S.E.lines; and from the manner in which buildings were thrown down, it was evident that the surface undulated in this same direction.("Geological Transactions" volume 6 pages 602 and 617."Journal of Researches" 2nd edition page 307.)CENTRAL AND NORTHERN CHILE.
Northward of Concepcion, as far as Copiapo, the shores of the Pacific consist, with the exception of some small tertiary basins, of gneiss, mica-schist, altered clay-slate, granite, greenstone and syenite: hence the coast from Tres Montes to Copiapo, a distance of 1,200 miles, and I have reason to believe for a much greater space, is almost similarly constituted.
Near Valparaiso the prevailing rock is gneiss, generally including much hornblende: concretionary balls formed of feldspar, hornblende and mica, from two or three feet in diameter, are in very many places conformably enfolded by the foliated gneiss: veins of quartz and feldspar, including black schorl and well-crystallised epidote, are numerous.Epidote likewise occurs in the gneiss in thin layers, parallel to the foliation of the mass.
One large vein of a coarse granitic character was remarkable from in one part quite changing its character, and insensibly passing into a blackish porphyry, including acicular crystals of glassy feldspar and of hornblende:
I have never seen any other such case.(Humboldt "Personal Narrative"volume 4 page 60, has described with much surprise, concretionary balls, with concentric divisions, composed of partially vitreous feldspar, hornblende, and garnets, included within great veins of gneiss, which cut across the mica-slate near Venezuela.)I shall in the few following remarks on the rocks of Chile allude exclusively to their foliation and cleavage.In the gneiss round Valparaiso the strike of the foliation is very variable, but I think about N.by W.
and S.by E.is the commonest direction; this likewise holds good with the cleavage of the altered feldspathic clay-slates, occasionally met with on the coast for ninety miles north of Valparaiso.Some feldspathic slate, alternating with strata of claystone porphyry in the Bell of Quillota and at Jajuel, and therefore, perhaps, belonging to a later period than the metamorphic schists on the coast, cleaved in this same direction.In the Eastern Cordillera, in the Portillo Pass, there is a grand mass of mica-slate, foliated in a north and south line, and with a high westerly dip: in the Uspallata range, clay-slate and grauwacke have a highly inclined, nearly north and south cleavage, though in some parts the strike is irregular: in the main or Cumbre range, the direction of the cleavage in the feldspathic clay-slate is N.W.and S.E.